A digital switching system is typically used by an organization for facilitating both internal communications within the organization and also communication with the outside world. These devices are often called Customer Premise Equipment (CPE), as the organization is typically a customer of the public telephone company. A DSS switches internal connections, such that a plurality of digital sets each connected to the DSS can communicate with each other, and also with the outside world via outside connections. A typical DSS will have outside connections which comprise analog connections to a central office (CO) switch for communication with the outside world via the Public Switched Telephone Network (PSTN), and connections to local digital telephone sets. Such a DSS allows a plurality of digital devices to share one or more analog lines. The DSS will convert between analog signals carried by the analog lines and digital signals used by the digital sets.
A DSS will typically only support digital devices. There are known advantages gained by using digital sets. However, one disadvantage of such a DSS is that analog devices, for example facsimile (fax) devices, modems, and analog telephones are not compatible with digital switching systems.
In the past, one way of using such an analog device was to have a dedicated analog line to the CO for use by the analog device. However, this has the disadvantage of being expensive to the customer, who has to pay for a dedicated line that will be underutilized, as the device is typically not used on a constant basis. Furthermore, as the line is dedicated, it can not be shared with other devices when the analog device is not using the line.
One prior art solution was to use an Analog Terminal Adapter (ATA). This is a device which is compatible with the DSS, and is considered by the DSS to be a digital device connected to the DSS. The ATA includes an analog line interface circuit which includes Digital to Analog converters for converting between digital and analog signals, as well as providing supervision (e.g., ringing voltage, off hook detection, etc.) to the analog device. Such a device is expensive and also has the disadvantage of not providing a true analog interface to the CO. Signal degradation can occur due to the combination of A/D conversion in the DSS and then D/A conversion in the ATA (and vice a versa). A user of an analog POTS (Plain Ordinary Telephone Service) telephone is unlikely to notice any effects arising from this double conversion. However, other analog devices, such as high speed modems (e.g., 56 Kilobit/second modems) will suffer signal degradation, which is unacceptable for data transmission and results in a decrease in data transmission rate. Furthermore, such a solution will not function in the event of a power failure (or at least will not function without battery backup).
Other prior art solutions include emergency transfer relay for connecting a POTS telephone to an analog line in the event of a power failure. Such a solution provides fall back to POTS in a relatively cheap manner, but does not allow for the use of a fax or modem. Furthermore, a call made during a power failure will be automatically disconnected when the DSS powers up and grabs the line.
Therefore, there exists a need for a system which will allow an analog device to share an analog line with a DSS which overcomes these problems in a cost effective manner.